Corrosion resistance can be imparted to steel parts, such as beams, girders, fencing, Re-bar etc. by galvanizing, i.e. coating the steel with zinc or a zinc alloy. One conventional process for galvanizing steel is the hot dip process. The hot dip process usually requires a pre-treatment step to remove scale and rust before the steel is coated with the zinc or zinc alloy. This pre-treatment step improves the adhesion of the zinc coating to the steel strip. Typically, scale and rust are removed by dipping in a hydrochloric acid solution often followed by rinsing in water. Parts are then dipped in an aqueous flux solution containing zinc-ammonium chloride and, in some instances, sodium fluoride and/or potassium, nickel or cerium chloride.
In the galvanizing step, the steel is dipped in a bath of molten zinc. After leaving the bath, the zinc coating is substantially smooth.
Many commercial applications of galvanized steel require that an additional finish coating or top coating be applied over the zinc coating. These top coatings are both for their aesthetic appearance, i.e. provide a choice of color, and to provide a durable maintenance-free top coat.
The top coatings must be specially formulated to ensure good compatibility, adhesion and uniformity of appearance when applied to the zinc coating. Even where the fabricators are cognizant of this fact and use the proper coatings, the coatings will soon commence to peel and crack.
Many fabricators simply spray the top coating on the zinc coating with little or no surface preparation. Without proper surface preparation and depending upon the ambient environment, surface peeling and rusting will quickly commence.
Other fabricators pre-treat the zinc coating, typically by sandblasting, before application of the final coating. This serves to `roughen` the surface. The roughened surface has an increased surface area to enhance the bonding of the coating to the zinc.
It has been found that with the prior art processes for preparing the zinc surface for the finished coating, typically by sandblasting, silica particles (impurities) become embedded in the zinc layer. These silica particles subsequently are oxidized and the oxidation reaction results in corrosion, i.e. cracking and peeling of the surface. That is, the prior art processes generally treat the zinc surface with materials which remain embedded in the zinc layer. These materials are impurities in the zinc coating and form oxidation sites which are the basis for the subsequent corrosion of the top coating.
The present invention is directed to a process for treating galvanized steel to prepare it for the application of a top coating. The process of the invention treats the surface of the zinc layer to `roughen` the surface without embedding impurities into the zinc. The invention comprises a method for preparing the galvanized steel for the top coating, the galvanized steel so prepared and the galvanized steel as finally coated.
Broadly the invention comprises a method for preparing galvanized steel stock for the application of a top coating. As is understood in the art, for galvanized steel there are typically four layers in the zinc coating. A first eta (.epsilon.) layer which interfaces with the steel surface, a zeta (.zeta.) layer, a delta (.DELTA.) layer and then finally a gamma (.gamma.) layer.
In the process of the invention, the zinc surface of the galvanized steel stock has at least an outer .gamma. layer and a .DELTA. layer adjacent to the .gamma. layer. The invention in one aspect comprises treating the zinc coating with zinc pellets to remove the .gamma. layer from the zinc surface while roughening the .DELTA. layer to provide a roughened grain-like surface. The coating is treated such that there is at least 15-40% more surface area available after treating than before. The roughness value (root-mean-square) is typically in the range of 1.5 to 4.0 .mu.m. Further, the zinc treatment step and application of the top coating are effected while the zinc coating is still malleable, typically within twelve hours after the completion of the galvanizing step. Lastly, the process ensures there are no impurities in the treated zinc coating. As used in this disclosure `impurities` comprise particles introduced into or formed in the zinc coating during the treatment step which particles would later form oxidation sites.